Historically, drilling operations rely on “after-the-fact” analysis to determine lithology, as well as other parameters. For example, in order to determine lithology, drill bit cuttings are physically analyzed at the surface by the wellsite geologist, or to be certain of a formations' lithology, physical parameters and geological facies, 30 feet core sections or sidewall cores are taken and analyzed at the surface. In each of the above methods, samples must be physically removed from the formation and returned to the surface for evaluation.
In order to provide real-time operational data, in-situ tools are used. The most popular is spectral analysis of the formation being drilled. Spectral analysis involves the interpretation of spectra obtained from the formation drilled using logging tools including a passive gamma ray detector and a neutron induced gamma ray log. In the latter tool, a neutron source is placed alongside the formation and periodically emits bursts of high energy neutrons to excite the atoms in the formation. A detector records the number of counts of returning gamma rays and segregates them according to their energies. The major drawback of this method is the recorded neutron induced gamma ray spectrums which are contaminated by significant background noise due to Compton scattering. Hence, spectral analysis cannot determine lithology as precisely as coring-based techniques.
Measurement-while-drilling is a type of well logging that incorporates downhole tools providing real-time information to help with steering the bit. These tools typically include sensors for measuring downhole temperature and pressure, azimuth and inclination, drilling mechanics information (e.g. torque, weight-on-bit, rotary speed, etc.) and a resistivity to determine the presence of hydrocarbons and water.
As the hole drilling operation progresses, a drill bit gradually degrades until it breaks. Replacing a drill bit after it breaks can be costly because of debris left in the hole that will need to be cleaned out. At the same time, deciding to pull the bit early results in lower bit utilization, increased operating costs, and lower productivity due to frequent bit changes.
Accordingly, there remains a need for improvements in the art.